Multimetal deposition method for detection of latent fingerprints: a review

Multimetal deposition is a versatile technique of detecting latent fingerprints on porous items, for example, fabric tape, masking tape, cardboard; on semi-porous items, for example, expanded polystyrene, latex gloves, waxed paper; as well as on non-porous items, for example, metals and plastics. It is so named because it is a two-step process, the first involves deposition of gold and the second deposition of silver on the ridges, thereby visualizing the latent impressions. Although a technique with a very wide range of applications, multimetal deposition is not very popular since the process of detecting fingerprints is quite cumbersome. In its modified form, called multimetal deposition II, some of the demerits of the original method are overcome. Multimetal deposition III and IV endeavor to further improve the performance of this technique. Yet another modification of the original multimetal deposition version is the fluorescent multimetal deposition a technique which produces a better contrast of the ridges vis-à-vis the background. Background A latent fingerprint is formed when the sweat pores of the papillary ridges leave a deposition of perspiration on a surface with which the finger has been brought into contact (Thomas, 1978). Human body possesses the following three types of glands – viz. eccrine, apocrine and sebaceous, the secretions of which contribute to a fingerprint deposit (Knowles, 1978). Eccrine glands are widely distributed throughout the body and are particularly numerous on the palms of hands and the soles of feet. These glands secrete chemicals as a result of general metabolism and catabolism. Besides water content, eccrine sweat contains up to 1% of the other substances of which inorganic salts constitute about one-half (Kuno, 1956). The other half is constituted by organic products like urea, creatinine, choline, lactic acid, sugars, uric acid, amino acids and proteins (Scruton et al., 1975). The multimetal deposition technique for detection of latent fingerprints is based on the interaction between these organic biomolecules, especially the amino acids and proteins with colloidal gold particles. The technique is so named because it involves the successive deposition of two metals on the * Correspondence: [email protected] Forensic Science Unit, S.G.T.B. Khalsa College, University of Delhi, Delhi 110007, India Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article International License (http://creativecommons.o reproduction in any medium, provided you giv the Creative Commons license, and indicate if macromolecule components of sweat residue and, in consequence, develops fingerprints on an array of crime scene evidence (Saunders, 1989b). The macromolecules, such as lipids, proteins and peptides present in the fingerprint residue are first stained with colloidal gold and then by a silver developer. The latter step amplifies the gold staining and enhances the clarity of ridge pattern. The typical laboratory procedure for multimetal deposition is carried out in several steps. The quality of developed fingerprints depends on such parameters as the size of colloidal gold particulates and pH of test solution (Jones, 2002). The dark gray coloured impressions are difficult to visualize if the background surface itself is dark or patterned. To overcome this problem, Becue et al. (Becue et al., 2006) recommended the use of gold colloids, dispersed in cyclodextrins as the staining solution. The modified method produced dark blue impressions and also reduced the number of operational steps. Subsequently, Stauffer et al. (Stauffer et al., 2007) replaced the silver staining step by gold nanoparticles. This not only eliminated one of the reagents, but also made the technique cost-effective. Mechanism It has been known since long that colloidal gold particles adhere to amino acids, proteins and peptides (Frens, 1973). The multimetal deposition technique of fingerprint is distributed under the terms of the Creative Commons Attribution 4.0 rg/licenses/by/4.0/), which permits unrestricted use, distribution, and e appropriate credit to the original author(s) and the source, provide a link to changes were made. Fig. 2 Structure of 3-ketoglutaric acid Sodhi and Kaur Egyptian Journal of Forensic Sciences (2017) 7:17 Page 2 of 7 development relies on the affinity of gold particles for these biomolecules, as these are present in fairly large concentration in sweat residue (Saunders, 1989a). The methodology is a two-phase process. In the first phase, colloidal gold particles adhere to the amino acid, protein and peptide content of fingerprint deposition, while in the second phase, silver particles get adsorbed on gold layer, improving the contrast (Holgate et al., 1983). The process was adopted from a sensitive immunogold-silver staining procedure, whereby the secondary colloidal silver deposition step improved the labelling of proteins (Champod et al., 2004). The colloidal gold particles are produced by the reduction of hydrogen tetrachloroaurate(III), commonly referred to as tetrachloroauric acid, by citric acid. Besides gold particulates, 3-ketoglutaric acid is one of the products. The structures of citric acid and 3ketoglutaric acid are shown in Figs. 1 and 2, respectively. The working solution of multimetal deposition method also contains sodium citrate, the structure of which is depicted in Fig. 3. The citrate anions, furnished by sodium citrate, engulf the colloidal gold particles and impart a negative charge to these, as is shown in Fig. 4. The amino acids (Fig. 5a; R is an alkyl or aryl group) or their fragments in proteins and peptides exist as zwitterions under neutral conditions (Fig. 5b). However, under acidic conditions, the amino acids acquire an overall positive charge, as shown in Fig. 5c. Citric acid brings the pH of the working solution in 2.5–2.8 range, thereby inducing a positive charge on amino acids present in sweat residue. Thus the colloidal gold particles bear a negative charge (Fig. 4) and the amino acids bear a positive charge (Fig. 5c). The electrostatic attraction between the two chemical entities is responsible for the adherence Fig. 1 Structure of citric acid of gold particles on amino acid, protein and peptide content of sweat residue. It is pertinent to state that citric acid performs two functions: Firstly, it reduces hydrogen tetrachloroaurate(III) ion to colloidal gold, and secondly, brings the pH of the working solution in the desired range. The gold particles adsorbed on amino acid, protein and peptide content of fingerprint residue do not impart a pronounced color to the ridges. Nevertheless, these provide a site for the nucleation of silver particles which, in turn, are produced by the reduction of silver(I) ions by iron(II) ions. Being a reversible reaction, there are chances that it may precede from right to left. However, the citrate ions complex with iron(III) ions and remove these from the equilibrium system, pushing the reaction to the right. The silver particles then settle on the colloidal gold, imparting a dark gray or black color to the ridges. A schematic diagram, showing the adsorption of gold on ridges, followed by adsorption of silver on gold is shown in Fig. 6. Preparation of reagents The following solutions are prepared for developing latent fingerprints by multimetal deposition technique.